Process of making shell molds

ABSTRACT

Apparatus for the formation of single crystal articles by directionally solidified casting techniques which substantially eliminates the formation of heterogeneous discontinuities in the casting.

United States Patent 1 Copley et al.

PROCESS OF MAKING SHELL MOLDS inventors: Stephen M. Copley, Madison;

Anthony F. Giamei, Middletown; Merton F. Hornbecker, Woodbury;

Bernard H. Kear, Madison, all of Conn.

United Aircraft Corporation, East Hartford, Conn.

Filed: Dec. 1, 1971 Appl. No.: 203,673

Related U.S. Application Data Division of Ser. No. 63,738, Aug. 14,1970, which is a continuation-in-part of Ser. No. 714,722, March 20,1968.

[73] Assignee:

U.S. Cl. 164/35, 164/60 Int. Cl. 1322c 9/04 Primary ExaminerR. SpencerAnnear AttorneyCharles A. Warren [57] ABSTRACT Apparatus for theformation of single crystal articles by directionally solidified castingtechniques which substantially eliminates the formation of heterogeneousdiscontinuities in the casting.

5 Claims, 6 Drawing Figures same m3 PAIENIEU JUN 1 9 ms M QQ 1 PROCESSOF MAKING SHELL MOLDS This is a division of US. Pat. application Ser.No.

63,738, filed Aug. 14, 1970.

BACKGROUND OF THE INVENTION bars it is desirable to produce such complexshapes as turbine blades and vanes with the entire blade or vaneincluding the root airfoil section and at times a shroud all of a singlecrystal The complex shaped castings have been limited in the past by theproblem of extracting heat at the proper rate since the removal of heatfrom the solidifying alloy has been accomplished primarily by theconduction of heat from the solidifying alloy through a constrictionadjacent to the bottom of the mold. The rate of heat removal through therelatively small cross-section of alloy in the constriction has been lowand therefore the thermal gradient and the rate of growth of the crystalhave been limited. One method for increasing the heat extraction ratehas been described in a co-pending U.S.- Pat. application, Ser. No.714,743, filed Mar. 20, 1968 now U.S. Pat. No. 3,543,284 for a processfor, casting single crystal shapes. This application has the sameassignee as the present application.

One of the problems in obtaining single crystal cast defect freearticles is the occurrence of jets which resuit in the formation oftrails of small equiaxed grains. Such convective jets can occur withinthe mushy zone during solidification. These jets lead to erosion andbreakage of the dendrite structure and thus cause the formation oftrails of equiaxed grains or freckles. The jets are a consequence ofhydrodynamic instability within the mushy zone and become morepronounced as the liquidus and solidus interfaces become curved or asthe mushy zonehe ight increases (i.e., the thermal gradient decreases).Freckle grains are a major defect since they detrimentally effect themechanical properties of the casting.

SUMMARY OF THE INVENTION One feature of this invention is an apparatusby which to cast single crystal parts such as turbine blades and vaneswithout the formation of these trails of equiaxed grains. Theelimination of these discontinuities in the surface is obtainedbyappropriate control of the thermal gradient within the mold, a controlof the rate of solidification of the alloy upwardly within the mold anda control of the wall temperature of mold surrounding the article beingcast. The latter resultis ob tained by surrounding the article mold atleast in part by a cavity in which a controlled solidification can takeplace ata selected rate thereby to more effectively control thetemperature of the article mold as solidification occurs.

In accordance with the, present invention, the article mold ispositioned within a surrounding control mold with the latter having oneor more casting cavities therein for toreceive the control portion ofthe hot alloy of the control of the alloy which is poured at the sametime as the alloy is poured into the article mold. These controlcavities are connected through solidification passages to the chillplate and the article tnold has a crystal selecting passage in the lowerend by which to cause the growth of a single crystal into the articleportion of the mold. Although the mold is adapted for the production ofa single cast article at one time the construction is more adapted forthe gang casting of a plurality of articles at one time and theconstruction is such that the solidified article may be readily removedfrom the solidified alloy in the adjacent and partially surroundingcontrol cavities.

The solidification process of the present invention is carried out bypositioning the one or several article molds within the surrounding moldand heating the en tire assembly to a temperature above the meltingpoint of the alloy. Once the alloy is poured in the mold assembly,directional solidification begins at the chill plate in which the moldassembly rests andthe liquidsolid interface moves upwardly from thechill plate in both the bottom of the article mold and in the connectorsfrom the casting cavities to the chill plate. The solidification of thealloy within the casting cavities or control cavities helps to maintainthe article mold at the desired temperature during; solidification andserves to control the rate of heat removal from the article molds sothat the liquid-solidinterface of the alloy within the article mold willbe retainedin asubstantially horizontal configuration during the solidifi cation of the alloy through the entire mold. This moldconstruction favors higher thermal gradients and growth rates needed forthe elimination of discontinuities such as trails of equiaxed grainsinthe cast surface. The rate of heat extraction by the metal in thecasting cavities is controlled to a substantial degree by the size ofthe connector from these casting cavities to the chill plate since areaof solidified alloy in these passages is a direct control of the rate ofheat transfer to the chill plate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a horizontal sectional viewthrough amold assembly.

FIG. 2 is a vertical sectional view substantially along the line 2-2 ofFIG. 1.

FIG. 3 is an elevational view of a portion of the mold assembly prior tothe formation of the second moldcoating thereon.

FIG. 4 is a view similar to FIG. 1 of a modified form.

FIG. 5 is a vertical sectional view substantially along the line 5--5 ofFIG. 4.

FIG. 6 is a fragmentary elevational view of the mold configuration ofFIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT known shell molding process inwhich thin mold shells are formed around disposable patterns asforexample wax patterns. The apparatus shows the mold shells after thewax has been melted out in order that the cavities in which the alloy iscast may be more readily distinguished. It will be understood howeverthat after the mold material has been completely formed around thepatterns they are melted out to leave the cavities shown and the waxpattern did, in fact, occupy all of the shown cavities prior to thebaking of the mold to harden it and to provide removal of the wax.

. The article mold 2 has a centrally located airfoil cavity 4 extendingfrom a root portion 6 to a shroud portion 8. Above the shroud is thefilling passage 10 and below the root 6 is the helix 12 that selects asingle crystal from the columnar growth base portion 14. The function ofthe growth portion 14 and helix 12 in selecting a single crystal hasbeen described in the copending application of Copley et al., U.S. Pat.Ser. No. 806,978 filed Mar. 13, 1969 now U.S. Pat. No. 3,625,275 andhaving the same assignee as the present application.

A plurality of these article molds are positioned in a ring with thechordwise dimension of the airfoil portion of the several article moldsextending in a generally cir cumferential direction as shown in FIG. 1.The several article molds are connected together in thecircumferentially arranged relation by a disk-like filler portion 16 forthe several article molds. This portion 16 has a chamber 18 thereincommunicating with all of the sev-' eral article molds.

Between adjacent article molds isa chamber 20 defined between inner andouter mold rings 22 and 24 which are in engagement with the surfaces ofthe article molds as shown in FIG. 1 leaving the trailing edge andleading edge portions of the mold exposed to the adjacent chambers 20.The chambers 20 are connected by passages 26 at the top communicatingwith the horizontal filling chamber 18 and the same chambers communicateat the bottom by a passage 28 with the chill plate 30 on which theentire mold assemblage rests. At the top of the filler portion 16 is apouring funnel shaped spout 32.

As above stated, the article mold 2 with the appendages thereon isformed by dip coating a wax or otherwise disposable'pattern that is theshape of the article together with the growth portion at the bottom andthe filling opening at the top. When the article mold is completed theseveral article molds are connected to a wax pattern that is the shapeof the chamber 18. Wax is positioned between the leading edge of onearticle mold and the trailing edge of the adjacent mold in the shape ofthe chamber 20 and wax rods are positioned at opposite ends which arethe shape of the passages 26 and 28. With all of the wax in position,the entire assemblage is again dip coated thereby forming the innerand'outer walls 22 and 24 togetherwith the mold l6 surrounding thechamber 18 and the portions of the mold connecting the mold portion 16to the individual article molds and also the portions of the moldextending down from the portion 16 to the top of the annular moldportions 22 and 24. Once the entire mold is created as shown, theassemblage is positioned in a suitable baking oven to harden the entiremold and to melt out the wax pattern thereby leaving a mold structure asshown in the drawings.

FIG. 3 shows an interim construction between the making of theindividual article mold 2 in this figure and the finished mold. In theshowing of FIG. 3 a wax pattern 34 has been positioned between adjacentarticle molds and wax rods 36 and 38 extend from top and bottom of thewax portion 34. At the top the rod 36 is attached to a wax disk 40 whichforms the chamber 18 in the completed mold. A wax pouring spout is alsoattached to the top surface of the disk 40 to shape spout 32.

Accordingly when the mold is completed ready to havealloy poured thereinfor the formation of the several cast articles the mold assemblyconsists of the circumferentially spaced apart article molds 2positioned between inner and outer rings 22 and 24 which definebetween'them the control chamber 20 the latter being an interruptedcircumferentially extending chamber since the article molds are incontact with the inner and outer mold rings and thereby break thecircumferentially defined chamber into individual segments which enclosethe trailing edge of one of the article molds and the leading of theadjacent article molds.

The entire mold assembly when it is ready to have alloy poured thereinis positioned within a heating device such as an induction furnaceincludinga cylindrical susceptor surrounded by an induction coil and themold is heated to a temperature above the melting temperature of thealloy in readiness for pouring. With the mold assembly resting on thechill plate 30, the alloy within the mold is in contact with the chillplate through the growth portion 14 of the article mold and through thepassages 28 leading to the control chamber segments 20. solidificationbegins at the chill plate and moves upwardly through the growth portionand through the helix where a single crystal is selected to grow intothe article forming portion of the article mold. Columnargrain growthextends upwardly through the passage 28 and into the chambers 20 and itis the solidification in these chambers and the absorption of the heatfrom the article mold into the solidified alloy and the chambers 20 andthe fence by conduction through the solidified alloy and the passage 28to the chill plate that permits a precise control of the solidificationrate and thereby the configuration of the mushy zone that is to say theliquid-solid interface of the solidifying alloy in the article molds. Bysuitable dimensioning of the passage 28 the rate of heat removal fromthe article mold may be so controlled that the liquid-solid interfaceremains substantially flat and horizontal during the entiresolidification cycle. A larger overall heat conduction path leads to ahigher thermal gradient.

Referring now to FIGS. 4, 5 and 6 the article mold 52 is similar to thatdescribed above and has the growth zone 54 at the bottom connected by ahelix 56 to the article forming portion and with an extension 58 at thetop of the article forming portion which serves as a place for fillingthe article mold. Several of these article molds are positioned in aring with the chordwise direction of the airfoil portion of the moldextending in a generally radially position rather than circumferentiallyas in FIG. 1. Inner and outer rings 60 and 61 define between them asubstantially annular chamber 64. The

inner and outer rings communicate and are in vertical 1 engagement withthe leading edges of the article mold by outward and inwardly projectingribs 66 and 68 formed on the inner and outer mold rings respectively.

As in the apparatus above described, there is a horizontal mold portion70 at the top of the mold assembly and this has a horizontal chamber 72communicating with all of the article molds and also through verticalpassages 74 in the mold with the segments of the annular chamber 64which are located between adjacent article molds. The mold also hasvertical passages 76 extending downwardly from the segments of theannular chamber 64 to engage at theirbottom ends with the chill plate 78as do the growth zones 54 of the article mold.

The mold assemblage is made in the same manner as above described withrespect to FIGS. 1, 2 and 3. A wax pattern for example is formed in theconfiguration of the article desired with a helical projection at thebottom and a configuration corresponding to that of the chamber in thegrowth zone portion of the article mold. The wax pattern has at the topan extension that serves to form the passage through the upwardextension 58 on the article mold and from there upwardly to communicatewith a horizontal wax disk the shape of and serving to form the chamber72 when the second step of making the mold is performed. When the waxpattern for the article mold is completed it is dipped into appropriatemold forming slurries by the usual technique of making the shell moldthereby forming the article mold 52 with the extensions at top andbottom as shown. This article mold is then assembled together with thewax pattern to form the chamber 72 wax patterns forming the passages 74and 76 and a ring of wax that forms the annular chamber 64 and enclosesthe article molds from top to bottom of the portion of the article moldthat becomes the finished cast article that is being cast for use.Suitable grooves are formed in this. wax pattern to communicate with theleading and trailing edges of the article mold thereby permitting themold forming material to contact the first shell when the assemblage isdipped to form the secondary mold coating.

FIG. 6 shows the assemblage of the article molds together with thesurrounding and associated wax patterns by which the secondary mold isproduced. As shown in this figure, the pattern assembly includes theflat disk 80 having the wax rod 82 extending downwardly into the articlemold and the parallel wax rod 84 extending downwardly to communicatewith the heavy wax ring 86 that surrounds and embeds the individualarticle mold for the effective portion of the article mold. Theassemblage also has the downwardly extending wax rods 88 from the waxring 86 down to a level coincident with the bottoms of the growthportions 54. Grooves 90 are formed in the wax and extend inwardly fromthe outer and inner vertical surfaces of the wax to expose the leadingand trailing edges of the airfoil portion of the article mold 52. It isthese grooves that permit the formation of theconnecting ribs 66 and 68.

The assemblage of FIG. 6 is then coated with mold material as by theusual shell molding process thereby forming the secondary mold whichsurrounds the wax pattern and assemblage of FIG. 6. This secondary moldhaving been completed'the device is placed in a furnace where the moldis baked to harden the material thereof to melt out the wax patternleaving the empty casting spaces as shown in FIGS. 4.and 5.

Once the mold is hardened ready for use, it is positioned within asuitable heating means preferably an induction furnace and preferablyall enclosed within a vacuum and the mold is heated to a temperatureabove the melting point of the alloy. The molten alloy is then poured tofill the spaces within the mold and solidification begins where thealloy contacts the chill plate in 6 the same manner as above describedwith FIGS. 1, 2 and 3.

With respect to both modifications of mold apparatus described, once thealloy is entirely solidified and cooled so that the mold with the alloytherein can be handled the outer or secondary mold is removed to asgreat an extent as possible and the molded parts of the casting arepulled apart. Because of the particular construction of the moldapparatus in both cases such that the control ring of alloy that is tosay the alloy that is formed within the segments of the annular chamber20 of FIG. 1 or the segments of the annular chamber 64 are separable onefrom another the cast segments may be readily withdrawn from one anotherthereby disengaging the cast control alloy from the article mold and thecast article within the mold. Subsequently removal of the helix of castalloy at the bottom of the article mold and from the removal of the castarticle from the upwardly extending portion of solidified alloyextending upwardly from the top of the article portion the latter isreadily removable in readiness for inspection and machining for use. I

With the use of the alloy in the annular chambers 20 or 64 and theeffective control of the solidification rate as well as the temperatureof the article mold during solidification of the alloy it is possible toobtain acceptable cast articles all of a single crystal and free of theequiaxed grains that tend to form on the surface when appropriatetemperature controls are not provided. Although the article produced bya casting technique of this type appears to be a relatively smallportion of the total amount of alloy used in making the casting it willbe readily understood that the remainder of the cast alloy the so-calledrevert may be added to the alloy for the subsequent casting operationsand is not in any sense wasted.

We claim: 1. The process of making a mold including the steps of forminga plurality of shell type article molds around disposable patterns eachof the latter having a filling extension at one end of the articleportion and a growth zone at the other end of the article por tion ofthe article mold, arranging said plurality of molds in a ring placing awax overlay between adjacent molds in the ring with the overlaysurrounding and embedding the effective portion of each of the adjacentarticle molds and filling the space therebetween, with a wax projectionforming a filling passage extending upwardly from said overlay andanother wax projection forming a downward extension on said overlayforming a second shell mold around the assemblage of article molds andwax inlay with said second shell mold in contact with the article moldswhere they are not encompassed by the wax overlay and heating the moldassembly to harden the mold and melt out the wax.

2. The process of making a mold as in claim 1 including the additionalstep of attaching a connecting wax connector element to the severalupwardly extending wax projections and to the wax filling extension onthe several article molds.

3. The process of claim I in which the article molds are shaped to forma turbine vane having an airfoil shaped portion with the added steps ofrespect to positioning the article molds in a ring with the chordpositioning the article molds in a ring with the chordof the airfoilshaped portion extending in a circumwise direction of the airfoil shapedportion extendferential direction and ing in a radial direction withrespect to the ring and embedding the leading and trailing edges of theairembedding the entire airfoil portion in the wax overfoil shapeportion of the article mold in the wax lay except for asmall portion atleading and trailing overlay. edges of the article mold.

4. The process of making a mold as in claim 1 in 5. The process ofmaking a mold as in claim 1 which the article molds are for themanufacture of a wherein the wax overlay extends for the entire lengthturbine blade or vane having an airfoil shaped portion of thearticleforming portion of the article mold. with the added steps of 10

1. The process of making a mold including the Steps of forming aplurality of shell type article molds around disposable patterns each ofthe latter having a filling extension at one end of the article portionand a growth zone at the other end of the article portion of the articlemold, arranging said plurality of molds in a ring placing a wax overlaybetween adjacent molds in the ring with the overlay surrounding andembedding the effective portion of each of the adjacent article moldsand filling the space therebetween, with a wax projection forming afilling passage extending upwardly from said overlay and another waxprojection forming a downward extension on said overlay forming a secondshell mold around the assemblage of article molds and wax inlay withsaid second shell mold in contact with the article molds where they arenot encompassed by the wax overlay and heating the mold assembly toharden the mold and melt out the wax.
 2. The process of making a mold asin claim 1 including the additional step of attaching a connecting waxconnector element to the several upwardly extending wax projections andto the wax filling extension on the several article molds.
 3. Theprocess of claim 1 in which the article molds are shaped to form aturbine vane having an airfoil shaped portion with the added steps ofpositioning the article molds in a ring with the chord of the airfoilshaped portion extending in a circumferential direction and embeddingthe leading and trailing edges of the airfoil shape portion of thearticle mold in the wax overlay.
 4. The process of making a mold as inclaim 1 in which the article molds are for the manufacture of a turbineblade or vane having an airfoil shaped portion with the added steps ofpositioning the article molds in a ring with the chordwise direction ofthe airfoil shaped portion extending in a radial direction with respectto the ring and embedding the entire airfoil portion in the wax overlayexcept for a small portion at leading and trailing edges of the articlemold.
 5. The process of making a mold as in claim 1 wherein the waxoverlay extends for the entire length of the article forming portion ofthe article mold.